The present invention relates to a fastener for coupling a medical device to a bone; more particularly to a bone screw; and most particularly to a bone screw that has a desirable pullout value.
Bone screws have long been known in the art. For example, U.S. Pat. No. 5,261,910 discloses the use of bone screws in conjunction with plates to fix bones into a specific spacial relationship. The methods and materials disclosed in U.S. Pat. No. 5,261,910 are hereby incorporated by reference.
Many prior art bone screws are single thread screws. See, for example, U.S. Pat. No. 4,950,270, hereby incorporated by reference. Such bone screws have a single ridge which travels along the screw in the shape of a helix. Double thread screws are also known. See, for example, U.S. Pat. No. 5,022,277, hereby incorporated by reference. An ordinary double thread screw has two ridges starting 180.degree. apart, both of which form helices along the length of the screw. Triple thread screws, wherein the ridges start 120.degree. apart, are also known. See U.S. Pat. No. 5,022,277.
The present invention comprises a bone screw which has increased pullout resistance by offering an increased thread depth while optimizing thread pitch. The bone screw of the present invention has a dual lead thread which is provided as an offset thread. Unlike ordinary double thread screws, in which the threads start 180.degree. apart, the bone screw of the present invention has two ridges which start closer together, illustratively, about 120.degree. apart. The ridges are provided in a spiral pattern, but appear to spiral together along the shaft of the bone screw, creating a large groove and a small groove. By providing an offset dual lead thread, machining limitations can be overcome to produce a bone screw with increased thread depth while maintaining or decreasing thread pitch, thus providing desirable pullout values.
In an illustrated embodiment, a plate and bone screw apparatus for orthopaedic applications is provided. The bone screw is provided with the offset dual lead and with a second threaded portion adjacent to a head of the bone screw. The plate is provided with a plurality of apertures having internal threads and the threads of the second threaded portion of the bone screw are positioned and sized to engage the internal threads in apertures of the plate. Preferably, the plate is not itself threaded. Rather, internal threads are located on expandable bushings provided within the apertures of the plate. Such a plate is disclosed in U.S. application Ser. No. 08/902,061, filed on Jul. 29, 1997, hereby incorporated by reference. Also, in the preferred embodiment, the shaft of the bone screw in the region of the second threaded portion may be tapered for locking into the expandable bushings.
Additional features of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.